125-200cc Head shake

Any info, help would be greatly appreciated on the following matter.
Since new I've noticed this husky 150xc likes to shake its head on certain occasions
Mainly fire road 5th - 6th gear if you hit a row of small woops or pot holes, the first few were ok then it would violently shake its head and turn into a tank slapper.
It was delivered with the forks protruding through the clamps to the 3rd ring, which made it very nervous so I moved it to the 1st ring from the top so about 5mm fork showing above triple clamp, calmed it down a bit.
Ended up getting a revalve for the Marzzochi forks and Sachs shock which seemed to get rid if the head shake, but have lost 10KG since the revalve, now down to 65kg without gear so probably 72kg - 75kg with gear.

The head shake is back in that same situation, have tried running between 100 - 103mm race sag which still sees free sag within spec.
Have reduced rebound on the forks thinking maybe they are packing since it only happens after a few in a row, never just two or three woops or pot holes.

Next will try reduce compression in the forks and put rebound back to where it was to see if that helps.

Can't really drop the forks so they are flush with the clamps as I'm only 5'5" and each fork ring move has made it harder to touch the ground, plus it slows the steering down to much in the tight st.
It's obviously a setup issue but trying to work out what it is is frustrating.
Perhaps now at my weight the stock fork springs are to firm?
Motosportz steering dampener would maybe cure it but they are underbar mounted and it would make the bar height to high for me.

Anyone have any suggestions on what to trial ?

sounds like you know your way around the bike...but make sure the head bearings are adjusted tight enough...

I wonder if rear rebound is too slow.

On some level, a bike that turns well in the woods is going to be a little bit "nervous" at the top of 6th gear. I'm not saying you can't improve the situation, but it's all about compromise.

On my Duc, the factory settings were super stable, but when I raised the rear (adjustable ride height) to improve turn-in, it started getting a bit of head shake when you unweight the front end. Worth the tradeoff!

Steering head bearing seems adjusted tight enough, the bars don't just flop & clunk side to side when its on the work stand.
I understand the light bike will get a little nervous at higher speeds but since the issue mostly went away once it was revalved and has only come back since I've lost weight it has to be a set up issue causing it.

Not sure if the rear rebound can cause such an issue
Perhaps I'll reduce the compression for the forks further and see what that does, then drop the forks flush with the clamps and have a hell of a time trying to touch the ground.

The only other thing that has changed is the tyre size, I've gone from bridgestone 403, 100/100-18 to a Pirelli XCMS 110/100-18 which is a little taller.

Taller rear tire (albeit only very slightly) and lighter rider (good job!)... you could be running a little higher in the rear than your sag numbers indicate. First thing I'd do is take a turn out of the rear preload and do the "better or worse" test. If you can't tell, take another turn out. That's free and easy! As are clicker changes. Two clicks more or less rebound... "better or worse."

Increase your sag a bit. Try 105-108mm and see what that does for you.

On the fork you want to increase the compression and decrease the rebound for the problem you are describing.

Picklito, Will def recheck the sag as I've probably lost another 3kg since last checking. Didn't even think to recheck it after going to a taller tyre.

Motorhead, you recommendation on adjusting the forks, I gather you think the forks packing? similar to what I thought it may have been doing.
Tuner had said the Huskies run best at 100mm race sag, but no harm in trying the 105-108mm you have recommended so I'll give that a go.

Thanks all for the tips, will take a note pad & tools out on the next ride and go back and forth on one section running through the above mentioned settings to try and iron the issue out.
If that fails, I guess it will be back for a revalve next year.

Also check swingarm and rear wheel bearings. Make sure you don't have too much compression in the forks. Back off compression till you bottom out softly on the biggest hits. You want the forks to respond to chop. My 2011 WR150 and 2013 WR125 /165 are both rock solid stable.

From what you said I was picturing some packing. So, you would want to back off the rebound to allow the fork to extend a little faster and try increasing the compression to keep them up in the travel a bit more. Sounds like they could be valved for more high speed or mid valve.

What year bike do you have?

Its a 2011 150xc.
Still has the stock fork springs but tuner set them at zero preload.
I had previously tried backing off rebound, but never altered the compression.
Will run through the range of suggestions above, and see what happens.

So, you have a 2011 WR150 with the stock fork springs set at zero preload, correct ?

What level is your riding ability ?

Zero preload is never correct setup for a fork IMO. Try 5-15mm and see if your situation improves. I get headshake in rough high-speed moto sections if I don't have my headset tension set pretty aggressive. My headset top nut also loosens which is enough to lower the tension on the bearings and make it handle more nervously, I now check it in my pre-race inspection and have found it loosening several times in 30 hours.